The information setting forth the placement of fuel bundles, each of which has various attributes, in a nuclear reactor core is referred to as the loading map. In conventional core design, creating the loading map is an experienced based, trial and error, iterative process.
The core designer generally receives plant specific critical to quality factors such as plant cycle energy requirements, thermal and operational limits, shut down margins, etc. The core designer will also have information on the layout of the reactor core; namely, an indication of the how the nuclear fuel bundles are positioned within the core. Some of the critical to quality factors may even concern the layout. For example, the core designer may receive input requiring the positioning of certain fuel bundles within the layout.
Given this information, the core designer then makes a guess, based on experience and various rules of thumb he may have developed over time, on the initial positioning of fuel bundles in the reactor core. Specifically, the core designer guesses how many fresh fuel bundles to place in the core, and what types of fresh fuel bundles to use. A fresh fuel bundle is a fuel bundle that has not been exposed. Fuel bundles of the same type have substantially the same attributes. The attributes include but are not limited to: uranium loading, average enrichment, gadolinia loading, number of axial zones, product line, and thermal-mechanical characteristics of the fuel bundles. Different types of fresh fuel bundles have one or more different attributes. In deciding how many fresh fuel bundles to use, the core designer is also deciding how many of the fuel bundles currently in the core to reuse. Reusing the fuel bundles currently present in the core can mean leaving a fuel bundle in its existing location, or moving the fuel bundle to a different location in the core.
As part of the core design, the core designer also determines other operational parameters of the reactor core such as control blade positions, core flow, etc. Having specified these operational control parameters, a Nuclear Regulatory Commission (NRC) licensed simulation program is then run on the initial core design. Based on the results of the simulation, the core designer utilizes experience and rules of thumb to fix perceived problems in the design and, in general, improve the design; particularly with respect to the critical to quality factors. These changes may include changing the loading map. The process repeats until the core designer is satisfied with the design.